The present invention relates to a liquid crystalline optical film useful in the fields of liquid crystal display, optoelectronics and optics.
A film having a fixed orientation form of a liquid crystal is a unique material which not only exhibits unique optical properties based on the structure of the liquid crystal but also exhibits a high resistance to environments. Generally, liquid crystals have large birefringences and various orientation forms and can display such optical performances as are unattainable by the use of a birefringent stretched film or the like.
Recently, various reports have been made with respect to properties of liquid crystals, particularly with respect to fixing oriented forms in order to utilize unique optical properties based on the oriented forms. For example, in JP64-65124A it is disclosed that a film with a twisted nematic orientation fixed is used as a constituent of a compensator for a liquid crystal display. In JP64-65124A is disclosed an optical filter comprising a smectic liquid crystalline polyester. Further, in JP6-186534 is disclosed a process for preparing a cholesteric liquid crystalline film. As is seen also from the above publications, films formed by fixing an oriented form of liquid crystal can greatly contribute to the technical development in the optical field, especially in the liquid crystal display field, by utilizing unique optical properties based on the oriented form of liquid crystal.
Because of such characteristics as low-voltage drive, light weight and low cost the liquid crystal display is occupying an important position as a substitute for the conventional cathoderay tube in the display field. The liquid crystal display makes display by utilizing an orienting behavior of a liquid crystalline substance having an optical anisotropy. This displaying method inevitably encounters the problem (hereinafter referred to as the "visual angle dependence") that on the display screen of the liquid crystal display there occur changes in color tone according to seeing directions. The visual angle dependence becomes more marked if a color compensating optical element such as, for example, a stretched film or a liquid crystal cell for compensation is attached to the liquid crystal display.
For example, the display of a personal computer or a word processor each utilizing a twisted nematic mode ("TN" hereinafter) or a supertwisted nematic mode ("STN" hereinafter) can afford a relatively good image when the display screen is seen from the front. However, when the display screen is seen from an angle other than the front, the screen may be colored or an image thereon may be difficult to see. Such visual angle dependence of the liquid crystal display is not liked by the user; besides, it gives rise to the problem that it is impossible to meet the requirement of a larger screen for a wall-mounted type large screen TV utilizing the liquid crystal display for example. More particularly, in the case of utilizing a large-screen display, even when the display screen is seen from the front, the marginal portion is seen at a certain visual angle due to the largeness of the screen, according to the prior art. Consequently, it is impossible to obtain a clear image throughout the whole screen.
As a compensating means for diminishing the aforesaid visual angle dependence of the display it has been suggested that the use of a film whose refractive index in the thickness direction is larger than that in a plane is effective M. Akatsuka et al.: Japan display '89, 336 (1989)!. Further, compensators which satisfy such condition are disclosed, for example, in JP5-27235A and JP5-53104A.
However, the compensators described in those publications are not satisfactory in point of performance although their visual angle dependence is improved over the conventional compensators. As one of causes, mention may be made of the fact that the orientation of liquid crystal molecules in a liquid crystal cell of TN or STN, more particularly a liquid crystal drive cell, has a twist structure and that the influence of the twist structure on the visual angle is not completely compensated even by the use of compensators described in the above publications. Thus, a further improvement has been desired.
According to a display method using a birefringence mode such as STN, twisted nematic liquid crystalline polymer films or various phase contrast films obtained by stretching plastic sheets are utilized in color display to make twist compensation for the twist structure of a liquid crystal cell. However, even by the use of such films for twist compensation it is impossible to solve the problem of the visual angle dependence. Under the circumstances, there has been a keen desire for the development of a compensating element capable of compensating both twist structure based on a liquid crystal cell and refractive index anisotropy.
It is an object of the present invention to provide a liquid crystalline optical film which has overcome the above-mentioned problems. It is another object of the invention to provide a compensating film for a liquid crystal display which film comprises the said liquid crystalline optical film. More specifically, the present invention aims at providing a compensating film capable of compensating both twist structure based on a liquid crystal cell and refractive index anisotropy in order to diminish the visual angle dependence of the liquid crystal display.